Insulating coating patterns are affixed onto a portion of the thin film conductive layer during two stages of printed wiring board manufacture, namely, in primary imaging for the definition of the printed circuit wiring pattern to be etched into the conductive layer, and also for the provision of a solder mask to be affixed permanently over a selected part of the printed circuit wiring pattern. Photopolymer coatings have been used for photodeveloping such insulation patterns from phototool images of the desired insulation pattern.
In the prior art processes, however, there are several unsolved problems. A most significant drawback is the lack of high resolution in the absence of high cost materials and processes, partly due to the tendency to trap air bubbles under polymer coatings, particularly in the case of dry film type photopolymers over metal conductor circuit traces extending typically 0.003 in. (0.008 cm) to 0.004 in. (0.01 cm) above the substrate laminate. Furthermore a dry film photopolymer is expensive, typically four times the cost of a liquid photopolymer. Also the force and high temperature necessarily used in the prior art to force the dry film over the conductor traces will also force the film into the circuit through holes, where not all the polymer can be removed in a normal washout process, thereby undesirably leaving the holes plugged. The dry film simply does not conform to sharp corners, etc. and tends to pucker so that a smooth surface is not achieved. Another serious disadvantage is that the dry film polymers do not adhere well to bare copper conductors, thus usually requiring an extra manufacturing step of pretreatment with a black oxide coating or the like. Also the energy cost and processing time and equipment cost is seriously increased because the dry film polymers need be heated and cooled for lamination.
When dry film photopolymers are used, they conventionally have the additional cost of a release layer covering of polyester or polyolefin plastic film on both sides, which must be removed and scrapped. Objectives of this invention include removal of all the foregoing disadvantages of the prior art usage of dry film photopolymers.
Other prior art problems occur in the use of photopolymers, including liquid photopolymers. One is the starvation of liquid photopolymer on the surface of the wiring traces extending above the printed wiring board substrate surface. Another is the necessity of processing the application of the polymer coatings in expensive and cumbersome large vacuum chambers to avoid trapped air. In large scale quantity manufacture also, the continual use of expensive precision phototool images deteriorates their resolution and thus the pattern production quality because of the abrasiveness of the repeated polymer coatings, etc. In this respect the phototool cost may be high because of replacement of the phototools every few hundred manufacturing cycles.
Particularly when using liquid photopolymers, there is a problem of "tenting" or bridging over holes in the printed wiring board because of extrusion into the holes to thin out the polymer covering layer. Also liquid polymers are not readily controllable for providing films of uniform thickness when thicknesses become substantial, such as those necessary to provide adequate thermal and electrical insulation for use as solder mask coatings. Furthermore with available polymers of acceptable commercial cost, in many cases the properties are not ideal for the comprehensive range of printed circuit wiring board functions. For example, the consistency for tenting holes of a liquid polymer having good adherence to bare copper wiring may not be good. Likewise the best electrical insulator and heat absorbent polymer for solder masking may not adhere well, etc. A further quality control problem for use of polymers, and particularly dry film polymers after taking off a protective release plastic film is the presence of lint and dirt particles on both the printed circuit board surface and adhering to the uncovered surface of the polymer, which affect resolution of photo developed patterns.
Accordingly it is an objective of this invention to find solutions to the foregoing problems and to provide manufacturing processes using polymers in the formation of insulation patterns over portions of a printed wiring board conductive film layer that are efficient in time, equipment and cost, and particularly efficient in the cost of expendible resources such as time, plastic films and polymers, all with an unexpected increase in resolution and performance. Other objects, features and advantages of the invention will be found throughout the following description, drawings and claims.